1. Field of the Invention
The present invention relates to a heat sink material for a heat sink which efficiently radiates heat generated by an IC chip, for example.
2. Description of the Related Art
Generally, IC chips are highly susceptible to heat, and should be designed such that their internal temperature will not exceed an allowable maximum temperature at a junction area of the IC chips. Semiconductor devices such as power transistors and semiconductor rectifiers consume a large amount of electric power per operating area, and cases (packages) and leads of such semiconductor devices may not be effective enough to sufficiently radiate heat generated thereby. Therefore, the internal temperature of those semiconductor devices tends to increase to such a level that the semiconductor devices may suffer a thermal breakdown.
The above phenomenon also occurs in IC chips incorporating CPUs. As the clock frequency of the CPU increases, the IC chip generates more heat while in operation. It is important to design IC chips to incorporate a thermally designed heat radiation structure.
Thermal designs for protecting IC chips against a thermal breakdown include device designs and mount designs which include heat sinks having a large heat radiating area and fixed to IC chip cases (packages).
The heat sinks are generally made of metal such as copper, aluminum, or the like that has good thermal conductivity.
Recently, IC chips such as CPUs, memories, etc. tend to be larger in size because of higher integration of devices and greater areas taken up by devices, while at the same time seeking a low power drive mode for low power consumption. IC chips of greater size are liable to peel off position or be mechanically broken due to increased stresses which are caused by the difference between the coefficient of thermal expansion of the semiconductor substrate (silicon substrate or GaAs substrate) and the coefficient of thermal expansion of the heat sink.
Possible approaches to the prevention of the above drawbacks include a low power drive mode for IC chips and an improvement of heat sink materials. At present, a practical low power drive mode for IC chips has a power supply voltage of 3.3 V rather than the conventional TTL level (5 V).
As for heat sink materials, thermal conductivity is not the only factor to be taken into consideration for their selection, but it has become necessary to select heat sink materials which have a coefficient of thermal expansion that is substantially the same as the coefficient of thermal expansion of silicon and GaAs which the semiconductor substrate is made of, and also have high thermal conductivity.
Various reports have been made with respect to improved heat sink materials. For example, proposed examples of heat sink materials include aluminum nitride (AlN) and Cu—W (copper-tungsten). AlN is suitable for use as a heat sink material for semiconductor devices which use a silicon substrate because it has a coefficient of thermal conductivity and a coefficient of thermal expansion well balanced and particularly because its coefficient of thermal expansion is substantially the same as the coefficient of thermal expansion of Si.
Cu—W is suitable for use as a material of a heat sink which has a complex shape because it is a composite material having a low coefficient of thermal expansion provided by W and a high coefficient of thermal conductivity provided by Cu, and it is easily machinable.
Other examples of heat sink materials include a ceramic base material mainly composed of SiC and containing 20 vol. % to 40 vol. % of Cu (conventional example 1: Japanese laid-open patent publication No. 8-279569) and a powder-sintered porous body of an inorganic material infiltrating 5 wt. % to 30 wt. % of Cu (conventional example 2: Japanese laid-open patent publication No. 59-228742).
The heat sink material according to the conventional example 1 is formed by powder molding that the heat sink material is molded a compressed powder of SiC and Cu into a heat sink. The coefficient of thermal expansion and the coefficient of thermal conductivity of this heat sink material are of theoretical values. Therefore, the theoretical values cannot be balanced against the coefficient of thermal expansion and the coefficient of thermal conductivity of actual electronic parts.
The heat sink material according to the conventional example 2 suffers a limitation on efforts to increase the coefficient of thermal conductivity as the ratio of Cu infiltrated into the powder-sintered porous body is low.
The above heat sink materials are suitable for use as composite materials having high coefficients of thermal conductivity. However, these heat sink materials fail to achieve a desired mechanical strength because residual pores are easily produced in the composite materials as a sufficient pressure for infiltrating the carbon material, such as a ceramic material for the semiconductor base, with the metal cannot be achieved for the reason that the mechanical strength of the carbon material is relatively low.
The residual pores result in a reduction in the coefficient of thermal conductivity of the composite material, an occurrence of voids at the interface between solder and the composite material in the step of soldering the composite material, and a reduction in the wettability between the composite material and the solder.
In recent years, it has become possible to produce a composite material having a high coefficient of thermal conductivity which is achieved by a carbon material only. As a result, the need for infiltrating metal into pores in a carbon material to produce a composite material having a high coefficient of thermal conductivity is decreasing. The metal serves the purpose of reducing residual pores to increase the mechanical strength of the heat sink material by efficiently infiltrating into the carbon material. Other tasks that need to be accomplished quickly include the prevention of soaking of water into residual voids that are produced in the plating step and the prevention of voids, bulging, and peeling which occur due to the water seepage.